1. Field of the Invention
This invention relates to a method of controlling the interfacial adhesion between a metal or alloy and an organic material, and to a structure produced thereby.
More particularly, this invention relates to a method of controlling the interfacial adhesion between one material, such as a metal or alloy, and another material, such as an organic material which has an affinity for absorbing water, water vapor or for absorbing or being degraded at the surface by any other fluid or solvent, and the coherent product formed thereby.
Most particularly, this invention relates to a method for improving the interfacial adhesion between a metal or alloy, particularly a film comprising chromium or chromium overlaid with copper, and a film comprising polyimide (PI). It also improves the polymer-to-polymer bond in a laminate structure. This invention relates also to the product formed as a result of the method, and the machine by which the product is produced.
The method, product and machine of the invention are particularly useful in the electronic packaging art, wherein polyimide films and coatings have potential as a flexible dielectric carrier, on which is disposed conductive circuitry, on which are in turn optionally disposed one or more of intergrated circuit chips and/or surface mounted components.
2. Background Art
Certain organic materials, particularly certain polyimides, are promising from many points of view for use as flexible film insulative carriers in electronic packaging applications such as, for example, in flexible and Tape Automated Bonding (TAB) configurations. Being flexible, polyimide is easy to handle and its coefficient of thermal expansion (CTE) is close to that of silicon, a fact which is convenient when connection is made between electronic chips and polyimide carriers. The coefficient of linear expansion of polyimide (2xc3x9710xe2x88x925 in/in degree C) is one of the best among polymers supplied in a roll format. Other properties of PI that make it useful for the electronics industry are its dielectric strength (5,400 volts/0.001 inch typical for 0.002 inch thick), its dissipation factor (0.0025 for 0.002 inch thick), its volume resistivity (8xc3x971015 ohm meters for 0.002 inch thick and 125 volts). However, the affinity of polyimide to absorb moisture creates a number of difficulties involved in its use, such as unreliable short and/or long-term adhesion between the polyimide and the conductive lines disposed thereon.
In the short term, additional processing steps subsequent to the initial deposition of metal-on-polyimide, such as steps related to additive electroplating, expose the polyimide to chemical solutions and solvents which are absorbed by the polyimide and provide a cause for lifting of the conductive lines. In the long term, a failure in the line adhesion due to temperature and humidity effects on a working device is intolerable in the high performance computer systems of the future, for which absolute reliability is necessary. Whereas the flexible film carrier with surface mounted chip or other device is a configuration which reduces problems due to thermal mismatch between the chip and the support on which the chip is mounted, the problem of adhesion between the metal and non-metal portions of the carrier has not been fully resolved.
It is known that polyimide absorbs about 3-4% by weight of water. However, exposure to water is unavoidable, as water is used in cleaning dust and debris as is caused by mechanical punching. Exposure to a humid working environment cannot always be avoided. In addition, the polyimide is exposed to water solutions of electrolytes in the additive plating process. When polyimide is exposed to an electrolyte, conductive ions from the solution are absorbed with solute, damaging the dielectric properties as well as the adhesive capability of the polyimide.
In U.S. Pat. No. 4,863,808 issued Sep. 5, 1989 to Sallo, the vacuum deposition of chromium as a barrier layer between copper and polyimide is described as affording resistance to undercutting by subsequent gold and tin plating baths. Applicants have found that the use of a chromium layer is not alone sufficient protection for polyimide.
Studies have proposed the role of carbonyl formation on adhesion promotion between metal and polyimide. There are basically two interpretations of the interaction between polyimide and a metal atom.
In the first approach, described in the IBM Journal of Research and Development, 32,658 (1988), by Ho et al., the formation of a charge transfer complex between the incoming metal atoms and the aromatic ring of the PMDA portion of the molecule is proposed. Quantum mechanical calculations suggest that the interaction above described is more important than the chromium-carbonyl group interaction.
In the second proposed interaction, described by Goldberg et al. in the Journal of Vacuum Science Technology, A6, 991 (1988), the authors describe the attack of the first Cr atoms on the PMDA portion of the polyimide structure. As a consequence of such an attack, the first Cr atoms become oxidized while the PMDA portion of the PI molecule becomes reduced to a mono-anion. Interaction of the carbonyl groups with the Cr ion induces formation of Crxe2x80x94O bonds. The first interaction involves a Cr ion with two carbonyl groups from different chains. Additional Cr deposition leads to formation of more Crxe2x80x94O and Crxe2x80x94N bonds.
The latter explanation applies to other metal/PI interactions besides Cr/PI. With an increased population of carbonyl groups, much greater interaction and better adhesion strength is expected at the metal/PI interface. This is why carbonyl group enhancement via oxygen plasma treatment may be expected to affect the adhesive quality of material produced.
However, the art neither describes nor suggests that the problem of adhesion between chromium and polyimide is solved by control rather than elimination of the water content of the polyimide, and the art does not describe or suggest the process steps, structure or device of the present invention. The art does not account for the fact that adhesion between metal and organic material does not simply bear a directly proportional relationship to the amount of carbonyl exposure, but actually diminishes beyond an optimum time of exposure.
The present inventors have made the surprising discovery that the removal of all water from the polyimide is neither necessary nor desirable, and that a small residual amount of moisture can actually enhance adhesion between the metal and non-metal to an extent not heretofore seen or expected.
Sputter roll metallizers are known in the art. However, in the systems known in the art, the oxygen DC glow chamber is found at the chill drum section. The sputter roll metallizer of the present invention is set inline apart from the DC glow chamber, allowing increased oxygen pressure.
It is therefore an object of the invention to provide a flexible film carrier having a controlled degree of adhesion between the polyimide base an d the circuitization layers.
It is a further object of the invention to provide a flexible film carrier having improved short and long-term adhesion between the polyimide base and the circuitization layer s which is superior to that known in the art.
It is a further object of the invention to provide a flexible film carrier with improved imperviousness to moisture whether in gas or liquid form.
The foregoing and other objects and advantages are accomplished in the present invention by means of process steps which include partial outgassing of water from the polyimide carrier at an elevated temperature to a predetermined, near zero level prior to surface treatment with energetic oxygen atoms. The process is performed in situ, optionally either in a vacuum sputter, cooled-roll system or as a panel process. The process leaves detectable xe2x80x9cfootprintsxe2x80x9d in the product which can be detected by Ruterford Backscattering (RBS).